In recent years there has been growing interest in the use of semiconducting polymers for electronic applications. One particular area of importance is organic photovoltaics (OPV). Polymers have found use in OPVs as they allow devices to be manufactured by solution-processing techniques such as spin casting, dip coating or ink jet printing. Solution processing can be carried out cheaper and on a larger scale compared to the evaporative techniques used to make inorganic thin film devices. Currently, polymer based photovoltaic devices are achieving efficiencies up to 7%.
A class of polymers that is currently achieving the highest efficiencies in polymer based photovoltaic devices is based on units that have a high quinoidal contribution. Poly(thiophene), for example, can exist in both the aromatic and quinoidal state as shown below:

A quinoidal structure reduces the torsion between adjacent rings, which results in a more planar polymer backbone leading to an extension of the effective conjugation length. It is generally observed in conjugated polymers that an increase in the conjugation length results in a decrease of the bandgap, leading to a higher degree of absorbed incident light.
The quinoidal state can be stabilised by fusing an aromatic ring to the thiophene backbone. The fused ring is itself only fully aromatic when the backbone is in the quinoidal state. This means there is a strong desire for the polymer to be in the quinoidal state. Previous work has demonstrated the use of a thieno[3,4-b]thiophene (1) [see Y. Liang; Y. Wu; D. Feng; S.-T. Tsai; H.-J. Son; G. Li; L. Yu, J. Am. Chem. Soc., 2009, 131 (1), 56-57], as shown below (wherein R is an alkyl group and Ar is an aryl group), to reduce the bandgap.

However, the synthesis of polymer (1) involves an unstable monomer, namely a 4,6-dibromothieno[3,4-b]thiophene, which complicates the polymer synthesis by leading to unpredictable polymer molecular weights that lead to varying polymer performance.
WO 2010/008672 A1 discloses semiconducting conjugated polymers of formula (2) and their use in OPV devices.
wherein R is polyfluoroalkyl, polychloroalkyl or an ester, R′ is alkyl, alkoxy, aryl, aryloxy, heteroaryl or heteroaryloxy, x is an integer from 1 to 12, and m is an integer from 1 to 200.
However, these polymers have limited opportunities for fine-tuning of the HOMO and LUMO energy level positions. Furthermore, the positioning of the alkyl side chains limits the utilisation of phenylene ring based comonomers, as such configurations would result in significant steric hindrance due to outward positioning of alkyl substituents on the thiophene rings.
There is still a need for organic semiconducting (OSC) materials that are easy to synthesize, especially by methods suitable for mass production, show good structural organization and film-forming properties, exhibit good electronic properties, especially a high charge carrier mobility, good processability, especially a high solubility in organic solvents, and high stability in air. Especially for use in OPV cells, there is a need for OSC materials having a low bandgap, which enable improved light harvesting by the photoactive layer and can lead to higher cell efficiencies.
It was an aim of the present invention to provide compounds for use as organic semiconducting materials that do not have the drawbacks of prior art materials as described above, are easy to synthesize, especially by methods suitable for mass production, and do especially show good processability, high stability, good solubility in organic solvents, high charge carrier mobility, and a low bandgap. Another aim of the invention was to extend the pool of OSC materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
The inventors of the present invention have found that these aims can be achieved by providing conjugated semiconducting polymers as claimed in this application.
The monomers and polymers of the present invention are especially suitable for large scale production. At the same time, they show good processability, high solubility in organic solvents, low bandgap, high charge carrier mobility and high oxidative stability, and are promising materials for organic electronic OE devices, especially for OPV devices.